The Magical MERLIN
One of the primary keys to the success of the Hawker Hurricane was the legendary Rolls-Royce Merlin 12-cylinder engine. Both the Hurricane and the Supermarine Spitfire had been designed around the Merlin and were the first to use it, but the V-12’s origins trace back several years earlier to the early 1930s. Rolls-Royce’s Kestrel V-12 had powered the Hawker Fury and numerous other aircraft of the pre-war period, but as the RAF contemplated moving away from biplanes to heavier, faster monoplane fighter aircraft, Rolls-Royce recognized that the 600-700 horsepower produced by the 21-liter Kestrel would be insufficient. Development began on the 27-liter Merlin, and by the time the first production engines (designated the Merlin II) went into Hurricanes in 1937, the engine was making more than 1,000hp.
Early Merlins made good power but proved less than reliable, necessitating a redesign of the cylinder heads and various other changes based on operational experience.
The availability of better quality (100 octane) fuel starting in 1939 permitted supercharger boost to be increased from 6 to 12psi for short periods of time, yielding a healthy jump in combat power to 1310hp. The 1940 introduction of a Stanley Hooker twospeed supercharger yielded the first major mechanical upgrade, allowing up to 16psi and 1,490hp from the Merlin XX, which was installed in the Mk II Hurricanes and Spitfires as well as Halifax and Lancaster bombers, among others. Numerous Merlin variants would follow, offering up improvements to reliability, special applications, and combat/ emergency power ratings as high as 2000hp before Merlin production ceased in 1950.
Starting in 1941, Rolls-Royce began licensing versions of the Merlin V-12 to be built in the United States by the Packard Motor Car Company, designated the Packard V-1650 Merlin. In total, Packard made in excess of 55,000 (more than a third of Merlin production!), many of which found their way into P-51 Mustangs and proved vital to that plane’s success as well.